For someone who has been making films as long as he has, there is a surprisingly short list of films where I would argue that Arnold Schwarzenegger gives genuinely good performances.
As a movie star, he doesn't really have to give good performances. That's one of the things that makes being a movie star so weird. There are legitimate legendary movie stars who have never given what I would call a good performance, but who do their jobs perfectly well. Being a movie star is far more about having a particular personality that you bring to every role. Most of the most famous Schwarzenegger films, he's just playing variations on himself. Even though I adore films like "Conan The Barbarian" and the first two "Terminator" films, I think his work in them is good because the directors of those films knew exactly what they wanted out of Schwarzenegger, and they practically build the films around him.